The present invention relates to lasers and, more particularly, to an improved laser which corrects for distortions in the wavefronts of the laser beam.
In an ideal laser resonator, with no aberrations in the lasing medium or distortions created by the reflecting means, only the fundamental resonating mode would be present. Since all the energy is contained in the single fundamental mode, no power would be lost, and optimum performance would be achieved.
However, there are problems associated with any laser device. Such problems include vibration of the reflecting surfaces, misalignment of the reflecting surfaces, heating of the reflecting surfaces thus causing warping, aberrations in the lasing medium, and turbulence in the lasing medium. These undesirable conditions result in lower system efficiency, and keep the system from performing at its diffraction limit, i.e., optimum focusing capability.
The optical components of the laser resonator govern the spatial coherency of the resultant laser beam and are determinative of optimum propagation and focusing capability. The above-mentioned undesirable conditions result in a reduction of the far-field intensity profile, and can require a reduction in the diameter of the mode-limiting aperture stop, thus reducing the extractable power.
Prior art solutions to these problems require a high degree of accuracy in the optical components used (typically fabrication accuracy to .lambda./10 or better), and mechanically stable oscillator cavities with low Fresnel numbers (typically a.sup.2 /.lambda.L.about.1). A mode-selecting aperture is conventionally used to select the lowest order transverse mode when optimum spatial coherence and beam propagation are desired. Accurate alignment of the focusing elements, such as the cavity mirrors, aperture, and the like is critical in the conventional laser resonator. A large mode diameter is generally desirable to achieve efficient extraction of laser energy using conventional plane or curved mirror laser resonators. This could be achieved only at the expense of even more stringent optical quality, alignment and lasing medium uniformity.
Another approach for producing a large mode diameter while providing better peformance involves the use of a spatial filter. This requires placing two lenses and a pinhole aperture within the laser cavity in the beam path. However, the disadvantages of this approach include additional elements which must be aligned, the same great sensitivity to optical aberrations of the medium or the optical elements, and the resultant loss of power upon the aperture. An additional difficulty is that high power operation is precluded by laser-induced breakdown at the aperture due to the presence of a tightly focused beam and high power density. This spatial filter approach has been largely abandoned in favor of unstable resonator designs.
Two prior techniques for minimizing alignment sensitivity involve the use of retroreflectors. Retroreflectors possess the property of reflecting a ray of light with an angular direction identical to the incident angle. The first uses a corner cube to eliminate the necessity of precise angular alignment of the primary reflecting element of the cavity (contrasted with the output element). The second uses a "cat's-eye" reflector, which is a lens and plane mirror combination, with the mirror located at the focal plane of the lens. The corner cube suffers the disadvantage of phase and polarization inversions about its three-fold axis of symmetry. The cat's-eye suffers from a power limitation, since it has a reflector located at the lens focus.
Prior attempts to correct unavoidable aberrations of the medium or optics have utilized a correction device external to the laser cavity. Two examples are the mechanically deformable mirror described in U.S. Pat. No. 3,731,103, and the technique of U.S. Pat. No. 4,005,935.
More recently, several attempts have succeeded in correcting phase front distortions in a laser cavity by using the mechanically deformable mirror inside a laser cavity. This technique is described in "Experimental Studies of Adaptive Laser Resonator Techniques", R. R. Stevens and R. C. Lind, with anticipated publication in Optics Letters, and "Adaptive Laser Resonator", R. H. Freeman, et al, Opt. Lett., Vol. 2, No. 3, March. 1978.
Drawbacks of this type of system include slow response times, need for external beam sampling to provide a feedback loop for the mechanical mirror servo system, and general system complexity resulting in high system cost and lower reliability.
Accordingly, it is an object of the present invention to provide an improved laser which corrects for aberrations in the laser wavefronts.
It is another object of the present invention to provide an improved laser in which the laser operates at or near its diffraction limit, that is, with optimum focusing capability.